Oral care compositions, such as toothpastes, may comprise metal ions as active ingredients. Zinc (II) ions and stannous (tin (II)) ions are perhaps the most important active metal ions. Stannous ions (Sn2+) are used in oral care compositions because they display biocidal activity and are effective in preventing dental erosion (Raklnatullina et al, 2013. “Inhibition of enamel erosion by stannous and fluoride containing rinsing solutions”, Schweiz Monatsschr Zahnmed, vol. 123, pp. 192-197). Zinc ions similarly display antibacterial properties.
In order to provide therapeutic activity, the metal ions must be released from the oral care composition into the oral cavity. Oral care compositions are typically contacted with the oral cavity for only a brief period before being rinsed away. The delivered dosage of metal ions is therefore dependent upon their rate of release from the composition. This must be taken into account when formulating oral care compositions. Accordingly, there is a need in the an for methods for measuring the rate of release of zinc and stannous ions from oral care compositions.
In addition, aqueous solutions comprising stannous ions have been reported to be unstable (Nevin el al, 1958. “Topical applications of sodium fluoride and stannous fluoride”, Public Health Rep, vol. 73, no 9, pp. 847-850). This is believed to he the result of oxidation of the stannous ions to inactive stannic (tin (IV)) ions. There is therefore a need for methods of measuring the rate of oxidation of stannous ions.